The present invention relates to a disc for recording and reproducing optical information, suited to use for various apparatus such as video and audio apparatus for recording and reproducing video and audio signals, office automation systems for filling large-volume data such as still pictures, documents and the like, and computers for processing and storing various information.
As is well known, a disc for recording and reproducing optical information has pits corresponding to the information recorded therein. In this disc, the minimum reflectivity of the pit is obtained when the pit depth is a quarter (1/4n) of the wavelength .lambda. of the light beam used. In a disc of the type in which the information is recorded in the recording tracks in the form of grooves and is read out from the same, the quality of the signals is impaired if the groove depth is selected to the .lambda./4n. In another known disc, the groove depth of the recording tracks is selected to be .lambda./8n, while the cue signal recording portion which is formed beforehand in the disc, e.g., the address signal portion, has a pit depth of .lambda./4n. This disc, however, is impractical in that the groove forming process requires two steps of depth control.